Find where the heat source is coming from. Excess heat is a common symptom of a short and considering it’s happening while the device is powered off and not charging it sounds like there’s a short on the main power lines. If you have a multimeter, set it to diode mode and touch the two ends together. When you hear a beep that means there’s a short. Now take the probes and probe the positive and negative pins in the battery connector ON THE MOTHERBOARD, NOT THE BATTERY ITSELF. Never test diode mode on a powered circuit. If you get a beep from touching the two pins together, there’s a short circuit.